Analysis of the Effectiveness of Grid Codes for Offshore Wind Farms Connected to Onshore Grid via VSC-Based HVDC

Transcription

1 Conference of the Wind Power Engineering Community Analysis of the Effectiveness of Grid Codes for Offshore Wind Farms Connected to Onshore Grid via VSC-Based HVDC Moritz Mittelstaedt, Andreas Roehder,.Hendrik Natemeyer, Prof. Dr.-Ing. A. Schnettler Institute for High Voltage Technology RWTH Aachen University

2 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 2

3 Motivation High share of offshore wind power (>2 GW targeted for 22 in German North Sea) Especially distant large Wind Farms are connected to Onshore-Grid via VSC-HVDC Source: TenneT GmbH Guarantee for a reliable, but also efficient energy supply High Wind Farm requirements, based on the demands in Onshore-Grid, applied to Offshore Wind Farms Unknown effectiveness of the Grid Codes, especially in Source: UK Offshore Wind Report 212 case of Offshore Wind Farms connected via VSC-HVDC 3

4 Central objectives Steady-State and fault behaviour investigation of a representative Wind Farm including a VSC-HVDC Model Presentation of the main studies on the effectiveness and possible simplifications of legal Grid Codes and requirements Motivation to discuss a possible modification of existing Grid Codes Who is responsible for System Services - Grid Operator or Wind Farm Operator? 4

5 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 5

6 Overview of relevant Grid Codes Grid Codes in Europe at a glance Legal Framework Germany German Ordinance on System Services by Wind Energy Plants (SDLWindV), Transmission Code 27 Spain Operation Procedures P.O Denmark Technical Regulation Great Britain The Grid Code Issue 5, Revision 3 (incl. Offshore part) Contents Classification of Wind Energy Units (WEU) regarding their size and connected voltagegrid level Requirements for the WEU in steady-state regarding e.g. Active/Reactive Power Supply Conditions for a disconnection from the Grid during faults Dynamic System Services during faults Purpose Necessary Flexibility for Grid Operators Avoiding cascading active power loss Fast and safe Return to stable Operation Point 6

7 Overview of relevant Grid Codes Requirements for German Offshore-WF Transmission Code 27 / SDLWindV Guideline of a PQ-Diagram for the Operating Points Voltage- and Frequency-Control LVRT-Capability, WEU have to stay connected to the Grid during faults and accomplish a contribution for system stability Control of reactive current injection by the WEU in relation to significant voltage deviation No specific requirements or exceptions for Offshore-WEU Grid Codes of the operators for a seaside connection Similar to requirements for the Onshore WEU Exception is only a different reactive current supply during faults 7

8 Overview of relevant Grid Codes PQ-Diagram for the WEU-Operating Points The PQ-Diagram defines the minimum obtainable Operation Area Three variations of PQ-Diagrams can be forced by the Grid Operator PQ-Diagram depends on Grid Voltage Depending on the Grid situation the Grid Operator can order to operate at specific points Onshore: Necessary flexibility to react on deviations in the grid 8

9 Overview of relevant Grid Codes Control of reactive current supply In case of a voltage deviation the WEU must back up the voltage by adjusting the reactive current I B reactive current deviation ( I B ) must be proportional to the relevant deviation, defined by the factor k For 3-pole faults e.g., WEU must be able to feed in a reactive current of min. 1% of the rated current active current I W can be reduced to obtain an increased reactive current additional time progressing requirements Source: System Service Ordinance SDLWindV 9

10 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 1

11 Model of an Offshore wind farm DFIG Model Generic Model of doubly-fed induction generator (DFIG) Turbine Chopper DC-Circuit with capacitor Representation of the mechanical behaviour as a oscillating Two-Mass-Model Implemented pitch control, Power-Frequency Control, Protection Systems e.g. Overvoltage- Protection, Crowbar-Protection LVRT-Capability Generator Crowbars Inverter Rectifier Filter Source: Perdana, Dissertation Chalmers Universitiy of Technology In model 5 Wind Turbines of 6 MW in 1 rows are applied Embedding in a 33 kv-offshore-grid 33/.69 kv Transformer 6,7 MVA 11

12 1 Model of an Offshore wind farm Overview of the model components Wind Turbines Internal grid and connection to the offshore converter DC-Transmission link Inverter Station with Wechselrichterstation und Connection Anschluss to an the das Transmission Übertragungsnetz Grid Rectifier Gleichrichterstation Station Sea Seekabel Cable G ~ Übertragungsnetz Transmission mit Grid GKW Onshore converter AC and DC Filter systems Windkraftanlagen Turbines Chopper Connection to the onshore grid, Representation by the first periphery

13 1 Model of an Offshore wind farm VSC-HVDC Control Scheme Wind Farm: Weak Grid Rectifier controls voltage amplitude and phase Rectifier works as reference machine Inverter Station with Wechselrichterstation und Connection Anschluss to an the das Transmission Übertragungsnetz Grid Rectifier Gleichrichterstation Station U ac, φ Sea Seekabel Cable U dc, U ac G ~ Übertragungsnetz Transmission mit Grid GKW Onshore Grid Windkraftanlagen Turbines Inverter controls V DC and V AC /Q Control of V DC and V AC independently 13

14 Model of an Offshore wind farm Reactive Power Control Wechselrichterstation und Anschluss an das Übertragungsnetz Realised Reactive Power Controls: Gleichrichterstation Seekabel Fix voltage or power factor control by every WEU at local bus Fix cos(φ) or Vbusbar (A power factor of cos(φ)=1 leads to an infeed of Q= MVAr at the 33kV Bus) Fix cos(φ) or Vbus Windkraftanlagen Fix voltage or power factor control by every row at the Central Wind Farm Busbar Reactive Power Supply depending on the Active Power Operation Point of every WEU to minimize internal Wind Farm losses Minimize PLosses 14

15 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 15

16 Stationary Analysis Selected control schemes Different control schemes disencumber the converter or the WEU electively Due to the VSC-HVDC the wind farm is totally decoupled from the Onshore grid regarding Reactive Power demand No impacts of voltage or load variations on the Wind Farm, contrary to the operation in Ohnshore grid, occur 1) x Q= LB 2) Q= WF BB Operation Points of the WEU, -,5 -,3 -,1,1,3,5 underexcited - Reactive Power Q [p.u.] - overexcited Operation Points of the Converter -,6 -,4 -,2,2,4,6 Reactive Power Q [p.u.] 1,2 1,,8,6,4,2 -,2 -,4 -,6 -,8-1 -1,2 16

17 Stationary Analysis Optimal Operation Points Dispersion of the given Active Power infeed P inside between the units ±5% Operation Points of the WEUs -,1 p.u. < Q <,2 p.u. Optimal power flow for minimum Losses in the Offshore wind farm Investigation for (n-) as well as for any possible combination of 1-3 Wind Energy Units outages More than 99% of all constellations are within a range of -,1 p.u. to,2 p.u. (n-) (n-1) (n-2) x (n-3) Possible downsized minimum reactive power supply range? 17

18 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 18

19 1 Dynamic wind farm analysis Fault in Offshore wind farm Fault at one Busbar in the wind farm Voltage drop to 4 % of nominal voltage Inverter Station with Wechselrichterstation und Connection Anschluss to an the das Transmission Übertragungsnetz Grid Rectifier Gleichrichterstation Station Sea Seekabel Cable G ~ Übertragungsnetz Transmission mit Grid GKW Feed-in of short circuit current by converter and WEUs Windkraftanlagen Turbines Fault clearing after 1 ms 19

20 Dynamic wind farm analysis Offshore wind farm fault - Results Voltage drop to 4 % nominal voltage successful clearing after 1 ms Fast increase of the voltage with overshot Reactive current supply by the WEU according to the k-factor Voltage settling time almost independent of the previous OP (WEU) Still significant impact by the additional reactive current injection (k-factors) Downsized DFIG current injektion has almost the same efficiency as a not downsized one [s] kv Onshore: Amplitude Voltage [p.u.] 155 kv Offshore: Amplitude Voltage [p.u.] [s].25 DC Onshore: Amplitude Voltage [p.u.] [s].25 DFIG: Reactive Power [p.u.] DFIG: Active Power [p.u.] 2

21 1 Dynamic wind farm analysis Fault in Onshore-Grid Fault at a bus near to the Point of common coupling (PCC) at the Onshore Grid Inverter Station with Wechselrichterstation und Connection Anschluss to an the das Transmission Übertragungsnetz Grid Rectifier Gleichrichterstation Station Sea Seekabel Cable G ~ Übertragungsnetz Transmission mit Grid GKW Voltage drop to % nominal voltage Windkraftanlagen Turbines Feed-in of the short circuit current mainly by the Onshore Grid Fault clearing after 15 ms 21

22 Dynamic wind farm analysis Onshore Fault without impacts on the wind farm Voltage drop to % at the Onshore Grid No more Active Power feed-in by the Inverter into the Onshore Grid DC voltages rises to threshold voltage Activation of the Chopper Ripple depends on chopper frequency and control, DC-capacitors and leak resistor Absorption of the surplus energy by the chopper of the HVDC-System Fault clearing after 15 ms No impact on the Wind Energy Units Absorption of the whole energy at the onshore station [s] kv Onshore: Amplitude Voltage [p.u.] 155 kv Offshore: Amplitude Voltage [p.u.] [s].25 DC Onshore: Amplitude Voltage [p.u.] [s].25 Chopper: absorbed Energy [MJ] 22

23 Dynamic wind farm analysis Onshore Fault with Fault Reflection Voltage drop to % at the Onshore Grid Controlled voltage drop at the offshore-side to 5 % nominal voltage with delay Reduced Active power feed-in by the WEU Acceleration of the generators and activation of the Pitch-control Delayed Re-feed-in of the whole active power due to the offshore voltage return Mechanical stress for the Wind Energy Units More than 6 % less absorbed energy by the chopper of the HVDC-System [s] kv Onshore: Amplitude Voltage [p.u.] 155 kv Offshore: Amplitude Voltage [p.u.] [s].25 DC Onshore: Amplitude Voltage [p.u.] [s].25 Chopper: absorbed Energy [MJ] 23

24 Content Motivation and objectives Overview of relevant Grid Codes Model of an Offshore wind farm connected via VSC-HVDC Exemplary results from the stationary analysis Dynamic wind farm analysis under different fault cases Conclusion 24

25 Conclusion and Outlook Presentation of a representative Model of an Offshore Wind Farm Examples for the application of the Grid Codes to Offshore Wind Farms connected via VSC-HVDC Possible modification of existing Grid Codes towards a downsized steady-state reactive power supply range for offshore wind turbine generators Significant contribution of a additional reactive current supply by the Wind Turbines Undefined Grid Codes for Offshore Wind Farms connected via VSC-HVDC leads to open question for manufactures, investors and grid operators about the need of abilities of the Wind Energy Units and the handling with faults ( Fault Reflection ) Special Grid Codes have to define the respective contribution to System Services by Wind Farm Operators and Grid Operators (Active power reduction) 25

26 Institute for High Voltage Technology RWTH Aachen University Dipl.-Ing. Moritz Mittelstaedt Phone: Fax: Thank You for Your Attention!